While anti-tumor drugs, generally aimed at controlling cell proliferation, are effective in treating fast growing tumors, paradoxically many primary breast tumors are slow growing but not necessarily indolent. Thus, novel molecular targets based on biological paradigms must be identified towards the elimination of such tumors. At the microscopic level, slow growing, well to moderately differentiated tumors can be readily distinguished from fast growing, poorly differentiated tumors and are assigned different histologic grades. However, the 'histogenetic evolution' of such differences remains elusive. We propose to use a novel approach to test the hypothesis that histologic grade is the combined manifestation of global genetic changes in the 'initiated' breast epithelial cells and the ensuing 'cross talk' with the surrounding stroma during tumor progression. A molecular road map supporting this model could assist in targeting tumors of all histologic grades. A major difficulty in the pursuit of this goal is the lack of cellular models for experimental manipulation, which reflect the breadth of biological heterogeneity in breast cancer. In our previous research, we designed and implemented strategies for establishing a comprehensive resource of live epithelial and stromal cultures from cancerous breast tissue. Here, we will use this unique resource to: 1. determine grade-specific gene expression in matched tumor-derived cellular subsets, using cDNA microarrays. 2. simulate biological interactions between these subsets to study paracrine regulation of proliferation, differentiation, and response to anti-tumor agents. 3. analyze grade-specific gene function in representative cell cultures by exogenous transduction and silencing approaches. 4. evaluate temporal acquisition of grade-specific gene expression within the tumor and contiguous blocks of adjacent non-malignant tissue. Completion of these studies will lead to an improved understanding of molecular signaling within and between cellular subsets resulting in primary breast tumors with varying degrees of histological and functional aggressiveness.